Tractor trailer combinations generally require a braking system on the trailer that is engaged when a braking system of the tractor is selectively engaged. Often the braking system of the trailer is coupled to the braking system of the tractor to allow the tractor to simultaneously apply the braking system of the tractor and the trailer. Conventional on-road trucks with pneumatic or air service brakes utilize a mechanical breakaway valve since the same operating pressures are used for air service brake and air trailer brake systems. On an agricultural tractor or work machine, however, the service brakes are hydraulic which utilize a much greater operating pressure than a pneumatic or air brake system.